1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission of a vehicle, and particularly, the present invention relates to a hydraulic control device for an automatic transmission that can switch line pressure of hydraulic oil between two steps.
2. Description of the Related Art
Heretofore, there is known a hydraulic control device for an automatic transmission that, when a load is applied to the automatic transmission, increases line pressure of a hydraulic circuit on the basis of the load compared with line pressure while stopping of the vehicle in order to improve fuel economy (gas mileage) of a vehicle provided with an automatic transmission (see, for example, Japanese Patent Application Publication No. 60-69356; hereinafter, referred to as “Patent Document 1”). In this vehicle provided with the automatic transmission, it is possible to prevent unnecessary consumption of fuel due to a so-called creep phenomenon while stopping of the vehicle particularly.
Specifically, to be explained with reference to a regulator valve 11 of FIG. 1, in the vehicle with the automatic transmission disclosed in Patent Document 1, an output of an engine is transmitted to a pump impeller of a torque converter via a crank shaft, and torque is amplified in a turbine impeller. A stator impeller shoulders reaction force of this torque. A stator arm 4b for controlling a regulator valve 11 in a hydraulic control device for an automatic transmission is fixed to this stator impeller, and stator reaction force is applied to a spring receiving tube 11b of the regulator valve 11 via this stator arm 4b. When the stator reaction force is increased, a stator spring 11c connected to the spring receiving tube 11b is compressed. The spring receiving tube 11b is moved to the left with this, and a set load for a compression coil spring 11a of the regulator valve 11 is caused to increase, whereby line pressure in a hydraulic oil passage is increased.
In the vehicle with the automatic transmission disclosed in Patent Document 1, by lowering line pressure while stopping of the vehicle, driving torque of an oil pump is reduced and friction torque of the automatic transmission is also reduced, whereby fuel economy of the vehicle can be improved. However, the line pressure can be increased using stator reaction force as described above, but it is impossible to further lower line pressure compared with normal line pressure, to which the stator reaction force is not applied, in a region in which a function and merchantability of the automatic transmission can fully be satisfied even under lower line pressure than normal. For this reason, there has been a problem that it is impossible to further improve fuel economy of a vehicle by further lowering line pressure in such a vehicle with the automatic transmission.
In order to solve this problem, it is thought to utilize a hydraulic control device that can switch (or change) line pressure between two steps by providing a solenoid valve for supplying auxiliary pressure to the regulator valve. FIG. 1 is a part of a hydraulic circuit of a hydraulic control device for an automatic transmission provided with a solenoid valve for supplying auxiliary pressure to a regulator valve. As shown in FIG. 1, this hydraulic control device for an automatic transmission includes: an oil pump P that becomes an oil pressure source; a regulator valve 11 for generating line pressure, which becomes original pressure of engagement hydraulic oil pressure for frictional engagement elements such as a clutch, by regulating base oil pressure of hydraulic oil supplied from this oil pump P; a solenoid valve 12 for supplying auxiliary pressure to the regulator valve 11 to switch line pressure between two steps; and a relief valve 13 that opens to reduce the amount of lubricating oil when oil pressure in a lubricating oil passage (hereinafter, referred to as “lubricating pressure”) becomes predetermined pressure. The relief valve 13 is provided in an oil passage 30 that is a lubricating oil passage at an output side of the regulator valve 11.
In the hydraulic control device for the automatic transmission shown in FIG. 1, by switching the line pressure to low pressure in a region in which higher engagement hydraulic oil pressure than that in an operation state of the vehicle is not required, driving torque of the oil pump P is reduced, and friction torque of the automatic transmission is also reduced, whereby improvement of fuel economy can be achieved.
However, as shown in FIG. 2B, when line pressure is set to low pressure, the amount of hydraulic oil leaking from the hydraulic control device falls compared with under high pressure. By lowering the amount of hydraulic oil in this manner, the amount of oil flowing into an oil passage 29, which is a lubricating oil passage, from the regulator valve 11 tends to increase. In particular, in a low input revolution region in which lubricating pressure is predetermined pressure or lower (the state where the relief valve 13 for reducing the amount of lubricating oil is closed), the amount of lubricating oil to clutches increases, and thus, friction torque of the clutches tends to increase. Therefore, even though a solenoid valve is merely added thereto to switch the line pressure to low pressure for the purpose of improving fuel economy, friction torque of the automatic transmission cannot be lowered in the low input revolution region as shown in FIG. 2A. Thus, there has been a problem that as a result, fuel economy cannot be improved in such a case.